1. Field of the Invention
The present invention generally relates to multi-piece golf balls and more particularly to golf balls having at least one component made of compositions comprising a polybutadiene rubber/ionomer blend. The golf ball includes an inner core (center) and at least one surrounding core layer. For example, the golf ball may contain a dual-core or multi-layered core comprising an inner core, intermediate core layer, and outer core layer. The golf ball includes a cover of at least one layer, preferably a cover having an inner and outer layer. Preferably, the center hardness of the inner core is greater than the surface hardness of the outer core layer.
2. Brief Review of the Related Art
Multi-piece solid golf balls having an inner core and outer cover with at least one intermediate layer disposed there between are popular today among professional and recreational golfers. In such balls, the inner core is made commonly of a natural or synthetic rubber, such as polybutadiene, styrene butadiene, polyisoprene, or highly neutralized acid copolymers. Often, the intermediate layer is made of an olefin-based ionomer resin that imparts hardness to the ball. These ionomer acid copolymers contain inter-chain ionic bonding and are generally made of an α-olefin such as ethylene and a vinyl comonomer having an acid group such as methacrylic, acrylic acid, or maleic acid. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. Commercially available olefin-based ionomer resins are available in various grades and identified based on the type of base resin, molecular weight, and type of metal ion, amount of acid, degree of neutralization, additives, and other properties. In conventional golf balls, the outer covers are made from a variety of materials including ionomers, polyamides, polyesters, and thermoplastic and thermoset polyurethane and polyureas.
Manufacturers of golf balls use different materials to impart specific properties and features to the ball. For example, the resiliency and rebounding performance of the golf ball is based primarily on the core. The core acts as an “engine” for the ball. In general, the rebounding performance of the ball is based on its initial velocity after being struck by the face of the golf club and its outgoing velocity after making impact with a hard surface. More particularly, the “coefficient of restitution” or “COR” of a golf ball refers to the ratio of a ball's rebound velocity to its initial incoming velocity when the ball is fired out of an air cannon into a rigid vertical plate. The COR for a golf ball is written as a decimal value between zero and one. A golf ball may have different COR values at different initial velocities. The United States Golf Association (USGA) sets limits on the initial velocity of the ball so one objective of golf ball manufacturers is to maximize COR under these conditions. Balls with a higher rebound velocity have a higher COR value. Such golf balls rebound faster, retain more total energy when struck with a club, and have longer flight distance.
Golf balls containing multi-layered cores are generally known. For example, Sullivan et al., U.S. Pat. No. 6,852,044 discloses a multi-layered core golf ball that comprises a center, a cover and at least two core layers formed around the center to create an inner ball, wherein the outermost core layer is relatively stiff and hard relative to the center. One outermost core layer is heavily filled with a density increasing material and at least one core layer functions as a moisture vapor barrier layer. Ohsumi et al., U.S. Pat. No. 5,772,531 discloses a golf ball comprising a solid core having a three-layered structure composed of an inner layer, an intermediate layer, and an outer layer, and a cover for coating the solid core. The intermediate layer is designed to have a JIS-C hardness of 50 to 80, and the outer layer is designed to have a hardness which is higher than the hardness of the intermediate layer.
One objective of the present invention is to develop compositions that can be used to make a highly resilient core for a golf ball. The ball also should have high durability and impact strength. The present invention provides golf ball compositions having such properties as well as other advantageous characteristics, features, and benefits.